Perhaps looking at this one from a different perspective would help. We know that:
Rate = Distance/Time
And they're asking us for the relationships between the rates of the two cars. It's then important to know that both cars will reach the same horizontal distance (as denoted by that dotted line) at the same times, so the times in the denominator of each relationship will be the same.
The difference is in the actual distance traveled, and we know that a 45-degree angle gives that distance a relationship of x to x(sqrt 2), based on the 45-45-90 triangle properties. So if the second car travels at a distance of (sqrt2) * Distance of the first car, then our Rate calculations are:
Rate (first) = Distance/Time
Rate (second) = sqrt 2 * Distance / Time
Then we know that the rates have a proportional relationship of x to x(sqrt 2), and we can calculate the percentage difference in their rates. That is why B is correct.